1. Field of the Invention
The present invention relates to exhaust gas emitted from a turbomachine and, more particularly, to a system and a method for cooling the temperature of the exhaust stream prior to recirculation of the exhaust stream into the turbomachine.
2. Discussion of Prior Art
The operation of turbomachines, such as gas turbines, involves the emissions of nitrogen oxides (hereinafter “NOx”), carbon dioxide (hereinafter “CO2”) and sulfur oxides (hereinafter “SOx”) and the allowable levels of these combustion by-products that can be emitted into the environment are heavily regulated. Exhaust gas recirculation (EGR) generally involves recirculating a portion of the exhaust gas stream through an inlet portion of the turbomachine where it is mixed with the incoming airflow prior to combustion. This process facilitates the removal and sequestration of concentrated CO2 and also reduces NOx emissions. The recirculated portion of the exhaust gas stream cannot simply be reintroduced to the inlet portion because the exhaust gas stream contains a variety of constituents, such as water, acids, aldehydes, hydrocarbons, sulfur oxides, and chlorine compounds, which, if allowed to enter the gas turbine untreated, will accelerate corrosion and fouling of the internal components.
Cooling the exhaust gas and performing scrubbing removes over 70% of the sulfur compounds present in the gas stream as well as significant quantities of the particulates. The cooling process may utilize tap water, water irrigated from a body of water or forced air in order to extract heat from the exhaust gas. However, such methods might not lower the temperature of the exhaust gas stream to the desired levels for recirculation. For example, there may be temperature requirements for the water and a plant site might not have access to such water. Moreover, in these methods, the heat in the exhaust gas stream is simply wasted and additional energy may be expended if a cooling process is implemented.
Thus, there is a need for a method that overcomes these difficulties in the cooling process and/or increases the overall electrical output of the overall system.